Cellular wireless communication networks are divided into a plurality of coverage regions. Each coverage region in the network is served by a Node-B. As a wireless transmit/receive unit (WTRU) travels, it may move from one coverage region to another in the network.
The WTRU is served by the designated Node-B for a particular coverage region. The regions covered by Node-Bs overlap each other, and at the boundary of the region a WTRU can establish connections with more than one Node-B. As the WTRU moves from one coverage region to another in the network, the WTRU goes through handover. Soft handover is widely used to ensure communication without interruption while roving around a plurality of cells.
Soft handover occurs when a WTRU is connected to two or more Node-Bs simultaneously, on the same frequency. In soft handover, all Node-Bs serving the WTRU process the received data, which is then routed to a radio network controller (RNC) for macro diversity combining. For simplicity, the RNC may use an error detection technique such as a Cyclic Redundancy Check (CRC) and may accept a packet that passes the CRC.
Softer handover is a special case of soft handover. When a WTRU is in softer handover, the WTRU is connected to two or more cells belonging to the same Node-B. In contrast to soft handover, in softer handover macro diversity with or without maximum ratio combining can be performed in the Node-B.
Automatic repeat request (ARQ) is a technique whereby the receiver requests a retransmission of packets by the transmitter if errors are detected. Hybrid ARQ (H-ARQ) is a technique whereby transmitted data blocks are encoded for partial error correction at the receiver, and only data blocks with uncorrected errors are retransmitted. In prior art, i.e. in high speed downlink packet access (HSDPA), the H-ARQ functionality is terminated and controlled by the Node-B, (a technique called Node-B-controlled H-ARQ), allowing for rapid transmissions and retransmissions of erroneously received packets. This feature was both highly desirable and practical because H-ARQ in HSDPA was not required for soft handover. This feature would be highly desirable for EU also, but problems exist because it is intended for EU (and H-ARQ) to operate during soft handover.
One of the problems with Node-B-controlled H-ARQ in soft handover is the link imbalance. Since the associated uplink (UL) and downlink (DL) control signaling does not benefit from the soft handover gain, it might be error prone and require significant power offsets. In the DL direction, the WTRU may not be able to receive the acknowledge (ACK) or non-acknowledge (NACK) signals from all involved Node-Bs. In the UL, not all involved Node-Bs may be able to receive the associated control signaling from the WTRU, which may lead to soft buffer corruption.
A soft buffer is a buffer for implementing H-ARQ in a Node-B. Data packets received, but not acknowledged, by the Node-B are temporarily stored in the soft buffer for incremental combining. Therefore, a data packet transmitted, but not acknowledged previously, is combined with a retransmission of the same data packet transmitted in response to NACK signaling. Chase combining is a special case of an incremental combining. The soft buffer corruption causes misalignment of an H-ARQ protocol state among different Node-Bs and leads to loss of the soft handover gain. It would be desirable to achieve efficient H-ARQ operation without the problems associated with prior art systems.
Node-Bs can often make more efficient decisions and manage UL radio resources on a short-term basis better than an RNC, even if the RNC retains overall control over Node-Bs. In order for a Node-B to assign UL radio resources to WTRUs in EU operation, the Node-B must know several WTRU-specific parameters. Under the current 3GPP standard, only the RNC can know the WTRU-specific parameters by means of radio resource control (RRC) messages. Therefore, it is necessary to forward the information to the Node-B for proper scheduling of radio resources in EU transmissions.
An RNC maintains an active set of cells for each WTRU in soft handover. The RNC bases its decision to add to or remove cells from the WTRU's active set upon measurements provided by a WTRU and a Node-B and on management of available radio resources in each cell. Under the current 3GPP standards, the RNC applies RRC radio bearer (RB) control procedures to coordinate active set cells with the WTRU, and Node-B application part/radio network subsystem application part (NBAP/RNSAP) radio link procedures to coordinate active set cells with each Node-B.
During soft handover, some information should be communicated between network entities to support EU operation. The information includes, but is not limited to, information related to an active set, information regarding a Node-B that controls transmissions during soft handover, EU scheduling information during soft handover, and ACK/NACK status information during soft handover. The current 3GPP standards do not define specific protocols to transfer necessary information which are imperative in operation of EU during soft handover. Therefore, it is necessary to define a protocol for transferring WTRU-specific information and other EU related information among an RNC, a Node-B, and a WTRU so that a Node-B is enabled to schedule radio resources and EU connections are handed over properly during soft handover.